1. Field of the Invention
The present invention relates to a radiographic image conversion panel using photostimulable phosphors, and to a method for manufacturing the radiographic image conversion panel.
2. Description of Related Art
Recently, methods for imaging a radiographic image by a radiographic image conversion panel utilizing photostimulable phosphors have been used.
As an example of such methods, there is the one using a radiographic image conversion panel in which a photostimulable phosphor layer is formed on a support (see, for example, U.S. Pat. No. 3,859,527 and Japanese Patent Application Laid-Open Publication No. Sho 55-12144).
Radioactive rays transmitted through a subject are irradiated to the photostimulable phosphor layer of such a radiographic image conversion panel to form a latent image (accumulated image) by accumulating the radiation energy corresponding to radiation transmittance of each portion of the subject in the photostimulable phosphor layer. Then, the radiation energy accumulated in each portion is converted into light by scanning the photostimulable phosphor layer with photostimulated excitation light (laser beams are used), and the intensity of the light is read to obtain an image. The image may be reproduced on various displays such as CRT and the like, or may be reproduced as a hardcopy.
It is required for the photostimulable phosphor layer of the radiographic image conversion panel used in the radiographic image conversion method to have high radiation absorptivity and optical conversion rate, to have good granularity of images, and to have high sharpness.
Generally, in order to make the radiation sensitivity high, it is required to make the layer thickness of a photostimulable phosphor layer thick. However, if it is too thick, there occurs a phenomenon such that luminescence does not come out because of scattering of photostimulated luminescence among photostimulable phosphor particles, and there is limitation.
Further, as for the sharpness, the thinner the photostimulable phosphor layer becomes, the more the sharpness improves. However, if it is too thin, the decrease of the sensitivity becomes large.
Further, as for the granularity, the granularity of an image is determined by local fluctuation of radiation quantum number (quantum mottle), by structural disarrangement (structure mottle) of the photostimulable phosphor layer of a radiographic image conversion panel, or the like. Therefore, when the layer thickness of the photostimulable phosphor layer becomes thin, deterioration of image quality occurs because the radiation quantum number to be absorbed in the photostimulable phosphor layer decreases and the mottle increases or because the structural disarrangement becomes obvious and the structure mottle increases. Accordingly, it is required to make the layer thickness of the photostimulable phosphor layer thick in order to improve the granularity of an image.
Thus, the image quality and sensitivity of the radiographic image conversion method using a radiographic image conversion panel are determined by various factors. Various studies have been made in order to improve the sensitivity and image quality by adjusting a plurality of factors relating to these sensitivity or image quality.
Among these, as a method for improving the sharpness of a radiographic image, for example, attempts for improving sensitivity and sharpness by controlling the shape itself of photostimulable phosphors to be formed have been made.
As one of these attempts, there is a method for using a photostimulable phosphor layer having a fine quasi-columnar block formed by depositing a photostimulable phosphor on a support having a fine concavoconvex pattern (for example, see Japanese Patent Laid-Open Publication No. Sho 61-142497). Further, a method for using a radiographic image conversion panel having a photostimulable phosphor layer in which cracks between columnar blocks obtained by depositing a photostimulable phosphor on a support having a fine pattern are shock-treated to be further developed (for example, see Japanese Patent Laid-Open Publication No. Sho 61-142500), further, a method for using a quasi-columnar radiographic image conversion panel in which cracks are generated from the surface side of a photostimulable phosphor layer formed on a face of a support (for example, see Japanese Patent Laid-Open Publication No. Sho 62-39737), furthermore, a method for providing cracks by forming a photostimulable phosphor layer having a void on an upper face of a support according to deposition, and thereafter, by growing voids according to heat treatment (for example, see Japanese Patent Laid-Open Publication No. Sho 62-110200), and the like are suggested.
Recently, a method for forming a photostimulable phosphor layer in a predetermined thickness by adjusting the intersection angle between the stream line of vapor flow of the photostimulable phosphor component and the support face in a specific range at the time of manufacturing the photostimulable phosphor layer on the support by using a vapor phase deposition method is disclosed (for example, see Japanese Patent Application Laid-Open Publication No. Sho 62-157600). Furthermore, a radiographic image conversion panel having a photostimulable phosphor layer in which an elongated columnar crystal having a constant slope to a normal line direction of a support is formed on the support according to a vapor phase deposition method (for example, see Japanese Patent No. 2899812 and Japanese Patent Application Laid-Open Publication No. Hei 2-58000) is suggested.
In these attempts of controlling the shape of a phosphor layer, it is a prospect to improve image quality by making a phosphor layer into a columnar crystal structure. Particularly, dispersion in the traverse direction of photostimulated excitation light (or photostimulated luminescence) can be suppressed by making the photostimulable phosphor layer into a columnar shape (can reach the face of a support by repeating reflection in a crack (columnar crystal) interface). Therefore, it is possible to remarkably increase the sharpness of an image by photostimulated luminance.
In these radiographic image conversion panels having a photostimulable phosphor layer formed by a vapor phase growth (deposition), the relation between the sensitivity and sharpness improves. Further, attempts such as suppressing reflection or refraction at the layer interface in a radiographic image conversion panel to further improve image quality by further combining a low reflective index layer with a phosphor layer including a quasi-columnar or columnar photostimulable phosphor crystals, and the like have been made (for example, see Japanese Patent Laid-Open Publication No. Hei 1-131498).
However, in a radiographic image conversion panel having a photostimulable phosphor layer formed by the above-described vapor phase growth (deposition), when a film is formed in a region that the temperature of a support is comparatively high, because elongated columnar crystals forming the photostimulable phosphor layer are formed on the support (substrate), there is a case that the adhesive property (adhesiveness) of the columnar crystals with the support is not sufficient, so that the phosphor layer is easily peeled off after the formation of the photostimulable phosphor layer. Therefore, improvement of durability has been required. Further, when a film is formed in a region that the temperature of a support is comparatively low, though adhesive property of a columnar crystal with the support is sufficient, the growth of the columnar crystal is not sufficient, so that the luminance and the sharpness decrease. Further, the lower the temperature of the support is, the more difficult it is to keep the temperature of the support uniform. As a result, unevenness of the image quality such as unevenness of luminance easily occurs. Therefore, improvement and uniformization of luminance and sharpness and improvement of adhesive property have been required.